Mercury generation

ABSTRACT

MERCURY RELEASING COMPOSITIONS EMPLOYING INTERMETALLIC COMPOUNDS OF MERCURY WITH ZIRCONIUM AND/OR TITANIUM SUCH AS ZR3HG AND TI3HG USEFUL TO CHARGE ELECTRON TUBES WITH MERCURY.

May 15, 1913 p, PORTA ETAL MERCURY GEN ERAT ION Original Filed Oct. '7, 1970 FIG1 United States Patent 01 fice 3,733,194 Patented May 15, 1973 US. Cl. 75-.5 R 4 Claims ABSTRACT OF THE DISCLOSURE Mercury releasing compositions employing intermetallic compounds of mercury with zirconium and/or titanium such as Zr Hg and Ti Hg useful to charge electron tubes with mercury.

CROSS REFERENCE TO RELATED APPLICATION This application is a divisional of US. application No. 78,839 filed Oct. 7, 1970, now US. Pat. 3,657,589.

Electron tubes containing mercury are well known in the art. In the past these tubes have usually been charged with mercury in its liquid form. However, such a procedure suffers from a number of disadvantages inherent in the storage and handling of liquid mercury due to its toxicity and other difiiculties inherent in the handling of a liquid metal.

There is present in the art a long felt need for an alternative to the use of liquid mercury. It has been proposed to introduce mercury into electron tubes in the form of a thermally decomposable compound of mercury. Examples of such prior attempts are disclosed in Rigot US Pat. 3,401,296 relating to the use of mercury pyrophosphate. Another procedure is by the use of a compound of mercury such as mercuric oxide and a reducing agent for the compound of mercury such as zirconiumaluminum alloy as described in Della Porta et al. US. 3,385,644. Unfortunately, the use of compounds of mercury suffers from a number of disadvantages such as the inherent danger of releasing noxious gases such as oxygen during mercury release.

The release of mercury after the tube has been evacuated and sealed is considered dangerous, if not impossible as the release of oxygen and other gases cause a loss of vacuum and other harmful effects within the tube.

Even if oxygen or other gases are released while the tube is being evacuated, due to mercury generation during the evacuation, the oxygen or other gases can still have harmful effects within the tube, on the electrodes for example.

To minimize the danger of release of oxygen it has been proposed to mix the mercury releasing compound with a non-evaporable gettermetal such as zirconium. The intended function of this getter material is to sorb the noxious gases which may be released concurrently with mercury release from the compound of mercury.

Another disadvantage of the use of mercuric oxide and a reducing agent is the relatively low temperature of approximately 250 C. at which the mercury releasing reductive reaction takes place. This relatively low temperature places an upper limit upon the temperature to which the electron tube can be raised during the degassing procedure which under conventional manufacturing techniques frequently precedes mercury release.

In an attempt to overcome the disadvantages inherent in the use of compounds of mercury it has been suggested in Keller et a1. U.S. Pat. 3,318,649 to employ an alloy of mercury with magnesium.

Keller also suggests the use of a ternary alloy of mercury, magnesium and nickel.

However, it has been shown that binary alloys of magnesium and mercury have generally proved unsatisfactory because of the low temperature at which mercury is released. When the mercury recombines with the magnesium undesirable gases which may have been sorbed by the magnesium can be released. Furthermore, evaporation of magnesium can take place at the low temperature at which the mercury is released.

The addition of nickel to form a ternary alloy as sug gested by Keller has led to only relatively limited im-' provements.

Another disadvantage of prior mercury vapor releasing composition is the relatively low weight percent, frequently less than 10 weight percent, of releasable mercury.

Accordingly, it is an object of the present invention to provide an improved mercury releasing getter device, an improved mercury vapor generating composition and an improved method of charging an electron tube with mercury all of which are substantially free of one or more disadvantages of the prior art.

Another object is to provide a means by which mercury can be generated in evacuated and sealed electron tubes which avoids the danger of gas release.

Another object is to provide a means for mercury generation which avoids the danger of concurrent oxygen release.

A further object is to provide means for mercury generation which does not limit the temperature at which high temperature degassing can be performed.

A still further object is to provide means for mercury generation employing a noxious gas sorptive non-evaporable getter material which after mercury release has a sufficient sorptive capacity to perform gettering functions throughout the life of the tube.

Yet another object is to provide a mercury vapor generating composition which has a high weight percent of releasable mercury.

Additional objects and advantages of the present invention will be apparent by references to the following detailed description thereof and drawings therein:

FIG. 1 is a top view of a mercury releasing getter device of the present invention.

FIG. 2 is a sectional view taken along line 2-2 of FIG. 1.

FIG. 3 is a top view of a modified mercury releasing getter device of the present invention.

FIG. 4 is a sectional view taken along line 44 of FIG. 3.

'FIGS. 5 and 6 are further mercury releasing getter devices. 7

FIG. 7 is a perspective view of a fluorescent lamp electrode employing a getter device of the present invention.

According to the present invention, there is provided a mercury releasing getter device comprising a holder and a mercury vapor generating composition carried by the holder, wherein the mercury vapor generating composition is an intermetallic compound of mercury and one or more metals selected from the group consisting of zirconium and titanium.

The preferred intermetallic compounds useful in the present invention are those of the formula:

wherein x and y have any value from O to 13 with the proviso that the sum of x and y is any value from 3 to 13 and z is 1 or 2. Examples of suitable compounds of the aforementioned formula include among others Zr TiHg, Zr Ti Hg, Zr Ti Hg Zr Hg, Ti Hg, as well as Zr Hg and Ti l-Ig. As described by Pietrokowsky in Journal of 3 Metals (February 1954) pages 219-226, Ti I-Ig has two crystalline forms, namely fiIi Hg and 'yTi Hg.

In the present invention both are suitable because the temperature at which they release mercury is high enough to permit degassing at high temperatures without danger of mercury release and still is low enough to avoid danger of melting or warping the holder.

The preferred intermetallic compounds employed in the present invention are characterized by having properties, such as for example, thermal stability, different from those that could be foreseen based upon the properties of the individual components. Furthermore, these intermetallic compounds have characteristic X-ray diffraction spectra. They can be produced by a variety of known procedures such as those described by Pietrokowsky, supra.

The intermetallic compound can be employed in any physical form such as a block, a strip or the like but is preferably employed as a finely divided particulate solid and generally that which passes through a US. standard screen of 10 mesh per inch and preferably that which passes through a screen of 70 mesh per inch. Even very fine particles such as those which pass through a screen of 600 mesh per inch can also be employed.

Although the above described intermetallic compounds can be employed alone, in another embodiment of the present invention they are mixed with a non-evaporable getter material. These non-evaporable getter materials are characterized by (l) a sorptive capacity for noxious gases such as oxygen, carbon monoxide, and water vapor, and (2) a vapor pressure at 1000 C. of less than 10 torr. Examples of suitable non-evaporable getter materials include among others zirconium, titanium, tantalum, niobium, vanadium and mixtures thereof, alloys thereof with one another and with other metals such as aluminum, which alloys have satisfactory gettering properties. The preferred non-evaporable getter material is an alloy of from to 30 and preferably 13 to 18 weight percent aluminum balance zirconium. The most preferred getter metal is one of 16 percent aluminum balance zirconium available from S.A.E.S. Getters S.p.a. Milan, Italy, under the trademark St 101.

The non-evaporable getter material can be employed in any suitable physical form but is preferably employed as a finely divided particulate solid such as one passing through a US. standard screen of mesh per inch and preferably that passing through a screen of 70 mesh per inch and being retained on a screen of 600 mesh per inch. In one embodiment the mixture of particulate mercury releasing intermetallic compound and particulate getter material is pressed into the cavity of an annular ring whereas in another embodiment this mixture is pressed onto a thin metallic substrate. The weight ratio of intermetallic compound to getter material can vary widely but generally is 100:1 to 1:100 and preferably 50:1 to 1:50. At greater ratios of mercury releasing compound the gas sorptive capacity of the residue is not substantial ly increased by the getter material. At lower ratios of mercury releasing compound the percentage of releasable mercury in the mixture decreases to an impractical level.

The holder can be in any physical shape which will carry the mercury vapor generating composition. In one embodiment the holder is an annular ring similar to that commonly employed to hold vaporable getter metals such as barium. In another embodiment the holder is a substrate which is preferably metallic and which has the particulate mercury vapor releasing composition embedded in at least one of its surfaces.

The same substrate may be used as a support for other materials which might be useful Within the tube such as getter materials.

In a further embodiment the holder is in the form of a wire or rod around which is formed a pill or pellet of the mercury vapor releasing composition.

The present invention is applicable to a wide variety of 4 mercury containing electron tubes examples of which include among other thyratrons, fluorescent light tubes, lasers, mercury rectifiers, various type of alpha numerical display tubes.

Referring now to the drawings and in particular to FIGS. 1 and 2 there is shown a mercury releasing getter device 10 of the present invention. In the getter device 10 the holder is in the form of an annular ring 11 having a cavity 12, and a mercury vapor releasing composition 13 Within the cavity 12.

Referring now to FIGS. 3 and 4 there is shown a getter device 30 which is connected to a similar getter device 30 which in turn is connected to yet another similar getter device 30". The getter devices 30, 30, 30", etc. form a continuous running length of devices. In the device 30' the holder is in the form of a substrate 31 having the mercury releasing composition 32 in particulate form partially embedded in the upper and lower planar surfaces of the substrate 31. In operation the getter device 30' for example is separated from the devices 30 and 30" by severing the substrate 31 in the vicinity of the small bridging attachments 33, 34, 35 and 36.

FIG. 5 shows a mercury evaporating getter device 50 in the form of a pellet in which the holder is in the form of a rod 51 having the mercury releasing composition 52 compressed around and supported by said rod.

FIG. 6 shows a mercury evaporating getter device 60 in the form of a pellet in which the holder 61 is a wire of high ohmic resistance in the form of a heating coil 62 around which is formed the mercury releasing composition 63.

According to another aspect of the present invention there is provided an improved method for charging an electron tube with mercury comprising the steps of inserting into the tube the above described mercury releasing compositions preferably by means of one of the above described getter devices and then heating the composition to liberate the mercury. The heating can be accomplished by any suitable means such as by radiation,

by high frequency induction heating, or by passing a current throughv the getter device when it is constructed of a material of high ohmic resistance. The heating is conducted at a temperature which will liberate the mercury from the mercury releasing composition. To a certain extent this temperature will be dependent upon the composition of the intermetallic compound. For Ti Hg and Zr I-Ig a temperature above 500 C. and preferably from 550 C. to 950 C. is suit-able. At temperatures much below 500 C. mercury is not released whereas at temperatures above 950 C. the release is so rapid that a danger of creating loose particles by thermal fracturing of the alloy exists. Another disadvantage of employing temperatures above 950 C. is the danger of undesirable noxious gas release from adjacent portions of the electron tube which tend to also be heated.

An important feature of the present invention is that the thermal decomposition of the intermetallic compound of zirconium and/or titanium with mercury leaves the Zirconium and/or titanium gas sorptive such that it functions as a getter metal throughout the life of the tube. The heating of the composition to release mercury is suificient to activate the getter metal.

Another important feature of the present invention is the ability to add other chemical compositions in mixture with the mercury releasing compound.

The invention is further illustrated by the following examples in which all parts and percentages are by weight unless otherwise indicated. These non-limiting examples are illustrative of certain embodiments designed to teach those skilled in the art how to practice the invention and to represent the best mode contemplated for carrying out the invention.

EXAMPLE 1 This example illustrates the synthesis of an intermetallic compound useful in the present invention.

Particulate titanium (143.7 g.) which passes through a standard screen with 400 mesh per inch is placed in.

EXAMPLE 3 This example illustrates the use of an intermetallic compound and a non-evaporable getter material.

The 'yTi -Hg (200 mg.) of Example 1 is mixed with St 101 alloy (200 mg). Both the Ti Hg and the St 101 alloy are of particle size such that they pass through a screen of 400 mesh per inch. The resultant mixture is pressed into an annular ring to produce a mercury releasing getter device similar to the device 10 shown in FIGS. 1 and 2, containing a coherent particulate composition 13.

This device is mounted in an electron tube and heated by surrounding the device 10 with a high frequency induction coil to heat the getter device 10 to 950 C. for 30 seconds to release at least 60 mg. of the mercury and activate the St 101 alloy.

EXAMPLE 4 This example illustrates the manufacture and use of mercury releasing getter devices similar to those shown in FIGS. 3 and 4.

A mixture of Ti Hg (100* g.) and St 101 (100 g.) is placed on a substrate of steel and pressed into the substrate as described in Italian Pat. No. 746,551 and US. application Ser. No. 33,794 filed May 1, 1970 to produce a strip of getter devices in which the mixture is distributed with a density of 30 mg./cm. similar to that shown in FIGS. 3 and 4 of the annexed drawings.

The getter device 30 is then placed in a vacuum tube which is then evacuated and the device 3 is heated to 850900 C. for 15 to 20 seconds to release mercury and activate the St 101 getter metal. The tube functions properly with respect to its mercury environment while continuing to sorb gases within the tube.

EXAMPLE 5 The procedure of Example 4 is repeated except that the slits forming the bridging attachments 33, 34, 35 and 36 are omitted and the resultant strip 31 formed into a circle around a fluorescent lamp electrode 70 as shown in FIG. 7.

EXAMPLES 6-8 Quantity of Compound Example number titanium, g. produced 6 191. 6 Ti4Hg 7 239. 5 Ti Hg 8 287. 4 TiaHg 6 EXAMPLE 9 This example illustrates the synthesis of a ternary intermetallic compound of the formula Ti Zr Hg- Particulate titanium-zirconium alloy (208.7 g.) having 34.1 percent titanium balance zirconium which passes though a standard screen with 400 mesh per inch is placed in a stainless steel crucible with mercury (200.6 g.). The crucible is then closed and heated to about 800 C. for about 3 hours. The resultant intermetallic compound when heated releases mercury at a temperature approximately C. higher than for either Ti Hg or Zr Hg.

Although the invention has been described in considerable detail with reference to certain preferred embodiments thereof, it will be understood that variations and modifications can be eifected within the spirit and scope of the invention as described above and as defined in the appended claims.

What is claimed is: 1. A mercury vapor generating composition of matter comprising:

(A). particulate Ti Hg and (B) a particulate alloy of 16 weight percent aluminum, balance zirconium, wherein the weight ratio of A:B is 50:1 to 1:50. 2. A mercury vapor generating composition of matter comprising:

(A) Ti Hg ((B) an alloy of 5 to 30 Weight percent aluminum, balance zirconium wherein the weight ratio of A :B is 100:1 to 1: 100.

3. A mercury vapor generating composition of matter comprising:

(A) an intermetallic compound of the formula:

wherein x and y have a value from 0 to 13 with the proviso that the sum of x and y is from 3 to 13, z is 1 or 2, (B) an alloy of 5 to 30 weight percent aluminum, balance zirconium, wherein the weight ratio of A:B is 100:1 to 1:100.

4. A mercury vapor generating composition of matter comprising:

'(A) an intermetallic compound of the formula wherein x and y have a value from 0 to 13 with the proviso that the sum of x and y is from 3 to 13 and z is 1 or 2, (B) a non-evaporable getter material characterized by: (1) a sorptive capacity for gases that have a deleterious effect on vacuum tubes, (2) a vapor pressure at 1000 C. of less than 10- torr, wherein the weight ratio of A:B is 100:1 to 1:100.

References Cited UNITED STATES PATENTS 3,385,644 5/1968 Della Porta et a1. 25281.3 U X 3,578,834 5/1971 Della Porta et al. 252181.6 X 3,579,459 5/ 1971 Della Porta et al. 252--181.3 X 3,401,296 9/1968 Rigot 252181.1 3,318,649 4/ 1972 Keller et al 316-3 OTHER REFERENCES Pietrokowsky Journal of Metals, vol. 6, pp. 219-26, February 1954.

OSCAR R. VERTIZ, Primary Examiner I. COOPER, Assistant Examiner US. Cl. X.R. 

